A few days ago, after changing my RODI sediment, carbon block, and DI and reading 1-2ppm after RO and 0ppm after DI, I discovered that I am still seeing 10-20ppb of phosphorus coming out of the RODI machine (that's over 0.06ppm at the top of the range).

How are you guys dealing with this? I am contemplating adding a second chamber of DI and hoping it'll catch it or also possibly running a chamber full of GFO after after my DI stage. For now, I am just blasting all of my water with LaCl and that seems to be working, but I would prefer not to use that if at all possible.

Personally,I wouldn't sweat 10-20 ppm phos. in my RODI. I would guess that you're using a Hanna ULR phosphate checker to get those kind of numbers. When you're testing in the ppb range the very slightest contamination can throw your results off significantly. There are many articles out there that say that minute amounts of phosphate actually benefit growth and coloration in acros and other corals as well as clams. I have seen reef tanks with phos readings in the .1-.2ppm range without any algae issues. If you really are concerned about your phosphate you could put some GFO after your DI cartridge and recheck the phos. before and after the GFO to see if there is a difference.

I agree with the absolute levels in the tank, the problem is I am adding almost 2 gallons (10% of my total water volume) of water at 0.05ppm of phosphate every single day with my ATO water (winter here is brutal compared to the steamy NC winter with 90% humidity). That increases my absolute tank levels by by 0.1ppm every month. Water changes really can't bring it down all that much either, since it can never be below 0.05ppm. I got some really nasty calothrix that took me more than a month to beat (and I am still seeing bits of) and it took me a really long time to figure out where the phosphates were coming from since I was starving the tank massively (which is terrible for my corals!). The much more expensive DI (the Spectrapure Silica-Buster Super DI at $29 a pop) seems to be really helping and it is usually under 3 or 4 ppb (for now). My ~20ppb water was using nuclear grade bulk DI media and it was only 5 months old (less than 400g through it).

I was just wondering if anyone else was having this problem - or if they aren't and have some mysterious phosphate in their tank out of nowhere, to tip them off to where it may be coming from.

I do use GFO to knock out the phosphates that do make it in on the RODI, but the massive amount of phosphates being dumped into the tank would exhaust the GFO after a very short period of time and the algae got out of control.

I've never bothered to monitor my phosphates over the years, and just change my GFO on a monthly basis without having any issues. What are you using to measure the phosphates? Also - I would agree that those levels are nothing to worry about. RHF has a good article talking about the different sources of phosphates here: http://www.advancedaquarist.com/2012/3/chemistry. The relevant paragraph from his article is here:

What about other sources of phosphate, like the "crappy" RO/DI water containing 0.05 ppm phosphate? A similar analysis will show it equally unimportant relative to foods.

Let's assume that the aquarist in question adds 1% of the total tank volume each day with RO/DI to replace evaporation. Simple math shows that the 0.05 ppm in the RO/DI becomes 0.0005 ppm added each day to the phosphate concentration in the aquarium. That dilution step is critical, taking a scary number like 0.05 ppm down to an almost meaningless 0.0005 ppm daily addition. Since that 0.0005 ppm is 40-600 times lower than the amount added each day in foods (Table 4), it does not seem worthy of the angst many aquarists put on such measurements. That said, tap water could have as much as 5 ppm phosphate, and that value could then become a dominating source of phosphate and would be quite problematic. Purifying tap water is important for this and many other reasons.

Note that is using an example of RO/DI where the numbers are 50 PPB, 2-3x higher than you were reporting. Although it doesn't address your original post, it's an interesting read if you haven't already seen it.

I am using a Hanna ULR phosphate checker (HI-736) to measure everything. I think my issues with phosphates in the RODI is due to the fact that I have such small volume, huge surface area and agitation (2x MP10, return, huge skimmer for my tank, filter sock in 20g+10g), and crazy low humidity (20% or less now) which leads to an absurd amount of topoff water. Back in NC I went through 2 gallons a week - now I go through almost that in a day (and it wasn't that bad in the summer)!!

I think that is the biggest issue. I did just increase my GFO, I run a little GFO when I am mixing my saltwater overnight, and the new DI cartridge seemed to have fixed my issues. I agree that 0.05 or 0.08ppm of phosphate is nothing to worry about (I normally aim for 0.03-0.05), but once calothrix took over, it became a huge issue and had to be dealt with. I couldn't starve out the algae breakout due to my RODI water. Once that was fixed, my algae issue steadily went away.

Good to hear that things are improving. I've never tested my phosphates, as previously mentioned, so I couldnt tell you what I have in my tank or in my RODI water. I evaporate closer to 5-7g per day in the winter, though, so I share your pain regarding lots of topoff water (although its all automated, so there's really no pain).

Have you been happy with the Hanna ULR tester? I'm not too familiar with it (though I have the Alkalinity tester) and am always interested in hearing about different testing devices.

BraenDead wrote:Good to hear that things are improving. I've never tested my phosphates, as previously mentioned, so I couldnt tell you what I have in my tank or in my RODI water. I evaporate closer to 5-7g per day in the winter, though, so I share your pain regarding lots of topoff water (although its all automated, so there's really no pain).

Have you been happy with the Hanna ULR tester? I'm not too familiar with it (though I have the Alkalinity tester) and am always interested in hearing about different testing devices.

Bob

My 2g is coming from around 20-25g of water though! My pico is the same exact way and I have never tested anything except for salinity on it and it is doing amazingly.

I really, really like the ULR checker. It isn't as convenient as the Alk checker since the reagent is a powder and you have to shake it like crazy for 2 minutes and then slowly invert it a bunch of times to get rid of all the bubbles that will throw off the result, but it is still way easier than almost all of the titration style tests.

It is pretty much the only phosphate test kit worth anything since it is the only one that is accurate (within reason) to the levels we have in our tanks. All of the color-code phosphate tests are useless for everything except for testing to see if your phosphates are completely out of control (and algae will tell you that long before you get there). I tested the checker against my Red Sea Pro and Salifert phosphate tests and the Red Sea Pro didn't show it's lowest reading (which is very close to "0") until an actual concentration of 0.09ppm and the Salifert didn't show it's lowest until 0.12ppm. RSP was reading 0.02 and Salifert was reading 0.03 at those measurements.

Just make sure if you do get it, you get the HI-736 - the phosphorus checker, not the phosphate checker since it is about 2.6x more accurate and they look the same!

I was looking at the Hanna web site and the HI-736 ULR reads phosphorus and the HI-713 reads phosphate. You would need to multiply the HI-736 readings by 3.3 to convert to phosphate. They also have calibration sets available to verify your readings. http://hannainst.com/usa/prods2.cfm?id=030011. Both are based on the same method.

BraenDead wrote:Interesting. Is the colorimeter reporting the concentration of Phosphorus (atomic mass 30.97), or of Phosphates (atomic mass 94.97) in the sample?

Bob

It reports phosphorus so you have to convert by multiplying your reading in PPB by 94.97 (95) then dividing the whole thing by 30.97 (31) then changing the magnitude by dividing by 1000 to get your reading in phosphate.

adadwa wrote:I was looking at the Hanna web site and the HI-736 ULR reads phosphorus and the HI-713 reads phosphate. You would need to multiply the HI-736 readings by 3.3 to convert to phosphate. They also have calibration sets available to verify your readings. http://hannainst.com/usa/prods2.cfm?id=030011. Both are based on the same method.

Yes, they both use the same method - however the reagents aren't interchangeable and it uses a different sensor- this translates to the the HI-736 being 2.6x more accurate. The HI-736 is +/- 5ppb phosphorus (0.015ppm phosphate) and the HI-713 is +/- 0.04ppm phosphate.

Very interesting - I wonder why they decided to call two of their meters Phosphate meters, and the ULR a Phosphorus meter - a marketing move to make it look like it reads lower than it actually does? It's still incredibly low ranges, but given they use the same method, I would think you would call them the same meter. Sorry for taking this discussion way off topic!

BraenDead wrote:Very interesting - I wonder why they decided to call two of their meters Phosphate meters, and the ULR a Phosphorus meter - a marketing move to make it look like it reads lower than it actually does? It's still incredibly low ranges, but given they use the same method, I would think you would call them the same meter. Sorry for taking this discussion way off topic!

Bob

It's no problem - The reason the ULR reads phosphorus instead of phosphate is just so they can display it on the screen with 3 digits since they would need an extra digit if they did it in PPM of PO4 since the resolution is 0.0033ppm.

Hanna does this with all of their meters and each "line" of meters has a variety of ranges that they test in, and they all overlap a bit. The ULR one has the highest precision and highest accuracy (1ppb resolution (0.003ppm PO4) and +/-5ppb (0.015ppm PO4)), but it's range is smaller than the regular Low Range Phosphate which has a resolution of 0.01ppm PO4 and an accuacy of +/- 0.04ppm PO4.

They also have a phosphate meter that reads in the PPM range only from like 1 to 200ppm with a resolution of 1ppm and it also uses the same method as the ULR and Low Range. It's just the sensitivity that changes.